Essential System Administration, 3rd Edition

Shutting Down a Unix System

From time to time, you will need to shut thesystem down. This is necessary for scheduled maintenance, running diagnostics, hardware changes or additions, and other administrative tasks.

During a clean system shutdown, the following actions take place:

• All users are notified that the system will be going down, preferably giving them some reasonable advance warning.

• All running processes are sent a signal telling them to terminate, allowing them time to exit gracefully, provided the program has made provisions to do so.

• All subsystems are shut down gracefully, via the commands they provide for doing so.

• All remaining users are logged off, and remaining processes are killed.

• Filesystem integrity is maintained by completing all pending disk updates.

• Depending on the type of shutdown, the system moves to single-user mode, the processor is halted, or the system is rebooted.

After taking these steps, the administrator can turn the power off, execute diagnostics, or perform other maintenance activities as appropriate.

Unix provides the shutdown command to accomplish all of this. Generally, shutdown sends a series of timed messages to all users who are logged on, warning them that the system is going down; after sending the last of these messages, it logs all users off the system and places the system in single-user mode.

Warning

All Unix systems—even those running on PC hardware—should be shut down using the commands described in this section. This is necessary to ensure ...